Tetrathiafulvalene (TTF) and its derivatives are molecules having strong electron donating properties, and form a charge-transfer complex together with an electron acceptor molecule such as tetracyanoquinodimethane (TCNQ). The resultant charge-transfer complex exhibits a metallic conductivity. The charge-transfer complex is expected to use in many applications, such as organic superconductors, organic magnetic materials, organic electrochromic materials and organic electroluminescence materials.
In recent years, thin-film transistors using organic semiconductors have attracted attention. Conventional processes for producing a thin-film transistor using silicon need vacuum conditions and a depositing step. Therefore, the conventional processes for producing a thin-film transistor have a disadvantage of requiring a very expensive production facility. However, a process for producing a transistor using the organic semiconductor can be reduced in cost, because of on-demand production of the transistor by a printing process using an ink formed by dissolving an organic semiconductor material in a solvent. Moreover, by means of the printing process using the organic semiconductor, an electronic circuit can be enlarged in area, or a flexible device can be produced.
It has been confirmed that a thin-film transistor using a TTF derivative, which has been conventionally reported, has a high field-effect mobility among organic semiconductors. In Non-Patent Literatures 1 and 2, by dissolving a TTF derivative in a solvent, a crystal is produced, and the crystal is placed between a source electrode and a drain electrode, so as to produce an organic semiconductor layer, and then transistor characteristics thereof is measured. It is confirmed that a thin-film transistor using DB-TTF as the organic semiconductor layer has a high mobility. However, a process for producing an element by placing a single crystal on the organic semiconductor layer of the thin-film transistor, is not practically employed as a process in which industrial convenience is considered. Additionally, typical TTF derivatives have a low ionization potential and poor atmospheric stability.
Non-Patent Literature 3 and Patent Literature 1 propose a molecular structure containing a nitrogen atom in order to improve an ionization potential of DB-TTF and DN-TTF. The DB-TTF and DN-TTF each containing a nitrogen atom is improved in ionization potential, but significantly decreased in mobility, by comparison with DB-TTF and DN-TTF, in which a nitrogen atom has not been contained.